Most oil and gas wells eventually require some form of stimulation to enhance hydrocarbon flow and make or keep them economically viable. The servicing of the oil and gas wells to stimulate production requires the pumping of fluids under high pressure. The fluids are generally corrosive and abrasive because they are frequently laden with corrosive acids and abrasive proppants such as sharp sand. Consequently, such fluids can cause irreparable damage to wellhead equipment if they are pumped directly through the spool and the various valves that make up the wellhead. To prevent such damage, wellhead isolation tools have been used and various configurations are well known in the art.
A general principle of wellhead isolation in the prior art is to insert a mandrel of the tools through the various valves and spools of the wellhead to isolate those components from the elevated pressures and the corrosive and abrasive fluids used in the well treatment to stimulate production. A top end of the mandrel is connected to one or more high pressure valves through which the stimulation fluids are pumped. A packoff assembly is usually provided at a bottom end of the mandrel for achieving a fluid seal against the inside of the production tubing or casing so that the wellhead is completely isolated from the stimulation fluids. The length of the mandrel need not be precise because the location of the packoff assembly in the production tubing or casing is immaterial so long as the mandrel is inserted into the production tubing or casing and a fluid tight seal is achieved between the production tubing or casing and the packoff assembly.
However, a packoff affixed to a bottom end of the mandrel which seals against the inside of the production tubing or casing, limits the internal diameter of the mandrel and, consequently, the flow rate at which stimulation fluids may be pumped into the well. To overcome this problem, applicant invented an improved mandrel for a wellhead isolation tool described in co-pending U.S. patent application Ser. No. 08/837,574 which was filed on Apr. 21, 1997 and entitled APPARATUS FOR INCREASING THE TRANSFER RATE OF PRODUCTION STIMULATION FLUIDS THROUGH THE WELLHEAD OF A HYDROCARBON WELL. The apparatus described in that patent application includes a mandrel for a wellhead isolation tool and a tubing hanger for use in conjunction with the mandrel. The mandrel includes an annular seal bonded to an outside wall above the bottom end of the mandrel. The annular seal cooperates with a sealing surface in the top end of the tubing hanger to isolate the wellhead equipment from high pressures and corrosive and abrasive materials pumped into the well during a well treatment to stimulate production. The novel construction of the mandrel and the tubing hanger eliminates the requirement for a packoff assembly attached to the bottom of the mandrel and thereby permits the mandrel to have a larger internal diameter for increasing the transfer rate of the production stimulation fluids through the wellhead. The axial length of the sealing surface in the tubing hanger available for packoff is limited and, therefore, the length of the mandrel is determined, to a large extent, by a distance from the top of the tubing hanger to the top of the wellhead.
Applicant describes another improved mandrel for a wellhead isolation tool in U.S. patent application Ser. No. 09/356,231 which was filed on Jul. 16, 1999 and entitled WELLHEAD ISOLATION TOOL AND METHOD OF USING SAME, which is incorporated herein by reference. The wellhead isolation tool includes a mandrel that is inserted into a wellhead. The mandrel is seated against an annular step above back pressure valve threads in a tubing hanger to isolate the pressure sensitive components of the wellhead from fluid pressure used in the well treatment and has a lower section extending past the back pressure valve threads and tubing threads into the tubing to protect the threads from washout. The annular step above the back pressure valve threads in the tubing hanger is a fixed-point for packoff of the mandrel and, therefore, a length of the mandrel is determined by the distance from the annular step to the top of the wellhead and a lockdown mechanism for securing the wellhead isolation tool to the wellhead preferably provides a range of adjustment to compensate for variations in the position of the top end of the mandrel when the mandrel is packed off in different wellheads.
Another example of a well tool in an operative position in which the mandrel of the well tool is packed-off against a fixed-point in the well is described in Applicant's U.S. Pat. No. 5,819,851 which issued on Oct. 13, 1998 and is entitled BLOWOUT PREVENTER PROTECTOR FOR USE DURING HIGH PRESSURE OIL/GAS WELL STIMULATION. The blowout preventer protector described in that patent includes a mandrel that is forcibly reciprocatable in an annular cavity of a spool. The mandrel is stroked down through a blowout preventer and packed off at the bottom end against a bit guide that is attached to a top end of the casing to protect the blowout preventer from exposure to fluid pressure as well as abrasive and corrosive well stimulation fluids. The bit guide attached to the top end of the casing provides a fixed-point for packoff of the mandrel.
It is apparent from the examples described above that, as a result of new tools being invented and new technology being developed, there is a need for a lockdown mechanism for securing a well tool requiring a fixed-point packoff in an operative position in the well.
The blowout preventer protector described in U.S. Pat. No. 5,819,851 includes a mandrel that is integrally incorporated with a hydraulic setting tool. The mandrel is not separable from the hydraulic setting tool and the setting tool is used to hydraulically lock the mandrel in an operative position. The mandrel. can be secured at any location within the annular cavity by maintaining the hydraulic pressure in the annular cavity after the mandrel is packed-off against the bit guide. The stroke of the hydraulic setting tool is used for inserting the mandrel through the blowout preventer, and also provides compensation for variations in a distance from the bit guide to the top of the blowout preventer when the mandrel is inserted through different wellheads. The blowout preventer protector is widely accepted in the industry and the hydraulic setting tool is very convenient for securing a mandrel of a well tool in the operative position requiring fixed-point packoff in the well. However, the setting tool must be fairly long to provide sufficient stroke. Furthermore, the setting tool is not removable from the mandrel during a well treatment to stimulate production. Consequently, the blowout preventer protector has a high profile. A well tool with a high profile is not convenient because access to equipment mounted thereto, such as a high pressure valve, is impeded by the height of the valve above ground. In addition, a hydraulic lockdown mechanism is considered less secure than a mechanical lockdown mechanism. The hydraulic lockdown mechanism is dependent on maintenance of the hydraulic fluid pressure in the setting tool. Since fluid pressure may be lost for a variety of reasons, persons in the industry are generally less inclined to endorse or accept a hydraulic lockdown mechanism.
A mechanical lockdown mechanism having a range of adjustment is used for the well tools described in Applicant's co-pending U.S. patent application filed on Jun. 23, 1999 and the application filed on Jul. 16, 1999 referenced above. The mechanical lockdown mechanism described in the above two patent applications is for securing a mandrel of well tools in an operative position requiring fixed-point packoff in the well, and provides a broad range of adjustment to compensate for variations in the height of different wellheads to which the well tool is mounted. The mechanical lockdown mechanism includes a base member that is adapted to be mounted to a top of the wellhead, the base member having a central passage to permit the insertion and the removal of the mandrel. The passage is surrounded by an integral sleeve having an elongated spiral thread for engaging a lockdown nut that is adapted to secure the mandrel in the operative position. The spiral thread on the integral sleeve and the lockdown nut have a length adequate to ensure safe operation at well stimulation fluid pressures. At least one of the spiral threads on the integral sleeve and the lockdown nut has a length adequate to provide a significant range of adjustment to compensate for variation in a distance between the top of the wellhead and the fixed-point for packoff in the well when the tool is mounted to different wellheads. The mechanical lockdown mechanism is separated from the hydraulic setting tool and, therefore, permits the setting tool to be removed from the well tool after the mandrel is locked down in the operative position. The tools therefore provide a low profile to facilitate well stimulation operations. The advantages also include the security of a mechanical lockdown mechanism. Therefore, there exists a need for a lockdown mechanism for securing a mandrel of a well tool in an operative position requiring fixed-point packoff in the well which provides a broader range of adjustment while ensuring a secure mechanical lockdown for maximum security.